1. Field of the Invention
The present invention relates to an electrode structure which improves an ohmic characteristic and a peel strength of a contact portion between a semiconductor and a metal electrode and which simplifies processing of the contact portion.
2. Description of the Related Art
Conventionally, a semiconductor device using III-V group compounds, such as a transistor, a diode, or the like, has a metal electrode for connecting the device to external electric circuits.
Electrode structures for the metal electrode stated above are divided broadly into two types, i.e., an alloy type and a non-alloy type. At first, an electrode structure of a semiconductor light receiving device used for optical communication will be explained below. There has been a rapid increase in demand for a semiconductor light receiving device in recent years.
FIG. 1 is a cross-sectional view showing an example of an InP/InGaAs-front-illuminated type semiconductor light receiving device (i.e., a photo diode).
The structure of this light receiving device will be explained as follows.
An n-type InP buffer layer 2 is formed on an n.sup.- -type InP substrate 1. An n.sup.- -type InGaAs light absorbing layer 3 is formed on the n.sup.- -type InP buffer layer 2. An n.sup.- -type InP cap layer 4 is formed on the n.sup.- -type InGaAs light absorbing layer 3.
A p-type impurity region 5 has a bottom portion positioned in the InGaAs light absorbing layer 3. The impurity region 5 is formed, for example, by selectively diffusing Zn (zinc) as p-type impurities into the InP cap layer 4.
In a light receiving device having the structure stated above, a so-called non-alloy type electrode is formed on a surface of the p-type impurity region. The non-alloy type electrode has a layered structure made of a Ti (titanium) layer 22, a Pt (platinum) layer 23, and an Au (gold) layer 24. The Ti layer 22 has a contact with p-type impurity region 5. Since the Ti 22 does not substantially react with InP contained in the p-type impurity region 5 to make an alloy, the non-alloy electrode stated above is formed.
A so-called alloy type electrode is formed on an InP substrate 1. The alloy type electrode has a layered structure made of an AuGe layer 25, Ni (nickel) layer 26, and an Au layer 27. The AuGe layer 25 has a contact with the InP substrate 1. The AuGe layer 25 reacts with InP of the InP substrate 1 to make an alloy, through a thermal treatment, and thus, an alloy type electrode which achieves an ohmic contact is formed.
In a non-alloy type electrode having a contact with the p-type impurity region 5, Ti layer 22 serves as a wiring layer for a mount or a wire-bonding, and a Pt layer 23 serves as a barrier 22 for preventing interactive diffusion between the Ti layer 22 and the Au layer 24.
The light receiving device stated above is mounted on a ceramic carrier, for example, by means of AuSn soldering, with the surface of the Au layer 27 being interposed as a connection surface. A wire of an AU line is bonded to the Au layer 24. A connection is thus obtained between the above light receiving device and external electric circuits.
However, the above light receiving device has defects as follows.
In a non-alloy type electrode having a contact with the p-type impurity region 5, a Schottky-barrier is easily formed on an interface between the Ti 22 layer and the p-type impurity region 5. As a result, a problem arises in that the Ti layer 22 and the p-type impurity region 5 cannot achieve a sufficient ohmic contact with each other, and the contact therebetween incurs a high resistance.
In addition, the Ti layer 22 which serves as a contact metal does not form an alloy together with InP contained in the p-type impurity region 5, but simply has a contact with the p-type impurity region 5. A peel strength is therefore insufficient between the Ti layer 22 and the p-type impurity region 5, so that the entire layered electrode consisting of layers 22 to 24 is easily peeled off from the light receiving device when an Au wire is bonded onto the Au layer 24. Consequently, the above light receiving device has a problem that the manufacturing yield and the reliability are low.
Meanwhile, as a conventional electrode structure, well-known is one in which an electrode having a contact with the p-type impurity region 5 is of an alloy type electrode in place of a non-alloy type electrode.
This alloy type electrode has, for example, a layered structure made of Au, AuZn, and Au or made of Au, AuCr, and Au, and Au has a contact with an InP cap layer 4. Since Au reacts with InP contained in the p-type impurity region 5 to form an alloy, through a heat treatment, so that an alloy type electrode is formed which can realize an ohmic contact.
Also, this alloy type electrode is characterized in that dopants, such as Zn, Cr, and the like, are introduced into an interface between the electrode and a semiconductor to achieve an ohmic contact which reduces a contact resistance between the electrode and the semiconductor.
However, Au tends to significantly react with InP to form an alloy layer having a high resistance. Therefore, there may be a case that impurities of Zn, Cr, and the likes do not effectively serve as dopants to lower the contact resistance, if a large amount of Au which is a main component of the contact metal reacts with InP to form an alloy.
A reaction of a large amount of Au with InP, as stated above, proceeds as time elapses. Therefore, there is a problem that, if an alloy type electrode is adopted in a planar type device whose electrode and p-n junction are close to each other, as shown in FIG. 1, the p-n junction is destroyed as the alloy-formation proceeds between Au and InP, and the reliability of the device is significantly degraded.
Further, in case of a conventional electrode structure, a p-type electrode (Au/AuZn/Au, Au/AuCr/Au, or the like) having a contact with a p-type impurity layer 5 and an n-type electrode (Au/AuGe/Au or the like) having a contact with an InP substrate 1 must be formed independently from each other.
Recently, developments have been made to a light receiving device of a structure in which a p-type electrode and an n-type electrode are formed on one common surface of a semiconductor substrate. In this kind of light receiving device, there is a problem that the process for forming electrodes is complicated and as a result manufacturing costs are increased, if the p-type and n-type electrodes have different structures.